Vacuum material lifters are typically attached to the end of the boom of an excavator, backhoe or other piece of large construction equipment. They are used to move large objects such as pipe, plate steel, traffic barriers and other large heavy objects that, in general, have a smooth, continuous and nonporous surface. These objects may weigh in a range of 0.1 metric tons to 0.8 metric tons (about 220 lbs to about 1,700 lbs) on up to 2 metric tons (about 4,400 lbs), 2 metric tons to 25 metric tons (about 4,400 lbs to about 55,100 lbs) or even up to 40 metric tons (about 88,100 lbs). The seal is fixed to the pad, as illustrated in FIGS. 1 through 4, and the lifter is configured to create a void space between the inner perimeter of the seal and the pad when the lifter is placed over the object to be lifted.
The operator of the equipment lowers the vacuum lifter until the seal on the outer perimeter of the pad comes into contact with the surface of the object to be lifted. Once the entire seal is in contact with the surface of the object, the operator opens a valve which creates a vacuum between the pad and the surface of the object. Once a sufficient vacuum pressure has been achieved, the operator can then lift the object using the boom of the excavator or other heavy equipment. The forces produced by the vacuum within the void space exert a compressive force on the seal which is relieved once the vacuum is released. The seal must have density memory to undergo many such compression/relaxation cycles without rupturing or exhibiting excessive permanent compression set in order to function optimally. The seal also must not exhibit a significant loss of stiffness during service otherwise the pad on the lifter might bottom out on the surface of the object to be lifted; thereby, preventing the development of an effective vacuum seal. It is obvious that maintaining an effective vacuum seal is critical to the function of the vacuum material lifter.
Seals for these vacuum lifters have typically been fabricated out of sheets of polyisoprene sponge or cellular rubber. These sheets come in various thicknesses and typically seals are comprised of two (2) plied thicknesses, each approximately 1 inch thick. Typical seals are fabricated into a 2″ wide×2″ thick cross-section, so in order to get the 2 inch thick seals needed for the vacuum lifters, it is necessary to laminate two layers of polyisoprene sponge rubber together. The lamination process typically involves the use of pressure sensitive adhesives, sometimes used with a binding layer of tape. Seals are typically fabricated such that the two (2) surfaces with exposed open cells are laminated together. The open cell surfaces are laminated together since the open cell surface provides the maximum amount of surface area for bonding. By laminating the open cell surfaces, the typical 2″ wide×2″ thick fabricated cellular seal will have two (2) surfaces with exposed open cells and two (2) surfaces with a rubber skin and no open exposed cells. The lamination process must be very well controlled in order to achieve adequate bond strength between the two plied layers. If the bond strength is less than the peel strength of the sponge rubber, the seal can delaminate or rupture in service which can result in the seal pulling out of the retention gland of the pad or loss of vacuum during lifting, causing the object to be suddenly released; thereby creating a potential safety issue. Seals are installed into the gland, as illustrated in FIG. 3, on the lower surface of the pad such that the laminated seam is aligned in the vertical direction to minimize the stresses at the laminated joint. Orienting the laminated seal in this manner causes the sides of the seal with the rubber skin to be on the vertical edges of the cross-section while the exposed cells are on the horizontal surfaces. This results in the seal having exposed cells on the surface that comes in contact with the object to be lifted. This causes the exposed cells to be susceptible to rapid abrasion and wear. The exposed cells will also quickly absorb liquids that can be on the surface of the object to be lifted.
One of the shortcomings of the prior art seals is that over time and particularly when seals are exposed to the elements, including elevated ambient temperatures and atmospheric ozone, the two bonded layers of sponge rubber may delaminate from one another causing the seal to fail. This occurs as the polyisoprene rubber and/or the bonding adhesive deteriorates causing the two layers to delaminate when subjected to compression loading cycles as the vacuum lifter operates. At that point, the seal, or at least that portion of the seal with the delamination, must be replaced. This adds to the downtime and the overall maintenance cost of the equipment.
Further, the prior art seals are susceptible to contamination by gas, diesel fuel, hydraulic fuel and other petro-chemicals commonly found on a construction site. These chemicals contribute to the deterioration of the polyisoprene rubber. This problem is compounded by the fact the seal does not have a continuous surface since the top and bottom surfaces of the seals have exposed cells. The exposed open cells of the laminated polyisoprene rubber seal may come into contact with a puddle of one of these chemicals, which can quickly be absorbed into the cells of the polyisoprene rubber causing rapid deterioration of the polyisoprene rubber. Once a seal has absorbed one of these chemicals into the cellular structure, it is impossible to remove all of the chemical or stop the deterioration of the seal.
What is needed, therefore, is a seal for a vacuum lifter that does not rely on a laminated seam. Further, what is needed, is a seal that resists absorption of liquids and provides greater resistance to environmental degradation with high abrasion resistance and improved resistance to permanent compression set.